Avoid racing on signal delivery with thread signal blocking in thread
groups.

The method to do this is to eliminate the per-thread sigmask_lock, and
use the per-group (per 'process') siglock for all signal related
activities.  This immensely simplified some of the locking interactions
within signal.c, and enabled the fixing of the above category of signal
delivery races.

This became possible due to the former thread-signal patch, which made
siglock an irq-safe thing.  (it used to be a process-context-only
spinlock.) And this is even a speedup for non-threaded applications:
only one lock is used.

I fixed all places within the kernel except the non-x86 arch sections.
Even for them the transition is very straightforward, in almost every
case the following is sufficient in arch/*/kernel/signal.c:

		:1,$s/->sigmask_lock/->sig->siglock/g